This invention relates to a cathodic electrodeposition paint forming films having improved surface smoothness. It also related to a cathodic electrodeposition paint forming films having improved surface smoothness and improved impact strength.
Amine-modified epoxy resins used as a film-forming cationic resin in cationic electrodeposition paints are known in the art, for example, in JP-A-54004978 and JP-A-56034186. These amine-modified epoxy resins are produced by reacting a bisphenol type epoxy resin with a primary or secondary amine to open all epoxy rings with the amine, or by reacting the epoxy resin first with a hydroxyl or carboxyl group-containing compound such as monophenols, monocarboxylic acids, hydroxycarboxylic acids, polycaprolactone diols or polyether diols to open a portion of epoxy rings and then with a primary or secondary amine to open the remaining epoxy rings. The resultant amine-modified epoxy resins are dispersed in an aqueous medium containing a neutralizing acid together with a crosslinker such as blocked polyisocyanates or melamine resin. Self-crosslinkable, amine-modified epoxy resins may be produced by reacting a bisphenol type epoxy resin with a half blocked diisocyanate to bind the diisocyanate through a secondary alcoholic group present in the resin molecule and then with a primary or secondary amine to open all terminal epoxy rings.
Cathodic electrodeposition paints derived from the above amine-modified epoxy resins have a defect, i.e., their wet films are susceptible to tiny craters induced by oil droplets or other particulate matters during the baking step resulting in cured films with impaired surface smoothness. Several methods are known to solve this problem including the addition of silicone anti-repellents or adducts of epoxy resin and polyoxypropylene diamine to the paint formulation. These additives, however, tend to decrease the adhesion of the paint film to a middle or top coating layer directly applied thereon.
A need exists for a cathodic electrodeposition paint which may produce a cured film having improved surface smoothness without imparing other film properties including adhesion to an overcoating layer.